FLUIDIZED BED COMBUSTION 403
maintaining the transport (entrainment) of the bed material
throughout the length of the combustor. The density of the
bed naturally varies with the combustor height, with density
increasing towards the bottom.
Steam may be produced at several locations. Water-walls
fi xed to the upper portion of the combustor extract heat gen-
erated by the combustor. The convective pass also emits
heat associated with the hot fl ue gas and solids which pass
through it. External heat exchangers are also employed for
the steam production. These heat exchangers (EHE’s) are
unfi red, dense fl uidized beds, which extract heat from the
solids which fall to the bottom of the cyclone(s). More than
one cyclone may be employed. The heat exchange is accom-
plished before the material is returned to the combustor. The
external heat exchanger is a device which can, thus, be used
as an effective additional method for controlling combustor
temperature. The heat transfer coeffi cients to the water-walls
usually lie between 20 and 50 Btu/hr. ft.^2 F.B. Bubbling Fluidized Bed CombustorsBubbling fl uidized bed combustors are characterized by
distinct dense beds. The bed material may be recirculated
as in the case of CFB’s, but at substantially lower recycle
ratios (between 2:1 and 10:1). Particle velocities are usu-
ally between 2–15 ft/sec and a small amount of bed material
is separated out (elutriated) as compared with CFB’s. The
mean bed particle size generally lies between 1000 and 1200
microns.
As with CFB’s, the fuel used is usually coal or some other
type of fossil fuel. Limestone or some other sorbent material
is also used to decrease SO 2 emissions. The feed material may
be fed either over the bed or under the bed. The manner of the
feeding is an important design criterion in that it effects boiler
control, emissions control (especially for SO 2 ) and combus-
tion effi ciency. Many bubbling bed designs incorporate over-
bed feeding in which the feed is “thrown” into the combustor
by pressurized air. This overbed method can often be a disad-
vantage because throwing distance is limited. Hence, a long,
narrow boiler is often required.
The underbed method of feeding is often associated with
plugging and erosion problems. However, these problems can
be avoided with proper design considerations. The Tennessee
Valley Authority (TVA) has designed a 160 MW bubbling
bed unit at its Shawnee station in Kentucky. The facility was
constructed at a cost of $232,000,000 (1989). EPRI believes
that most retrofi ts would fall into the $500–1000/kW range
(1989 dollars) and that the levelized generation cost would
be 5–10% less than a conventional unit with downstream fl ue
gas treatment. 1a The coal used for this unit is crushed to less
than ¼ inch and dried with fl ue gas to less than 6% moisture.
The fuel then passes through a fl uidized bottle splitter with
a central inlet and fuel lines arranged concentrically around
the inlet. The feed material is forced into the combustor from
the bottles, which are pressurized, by blowers. Each bottle
acts as an individual burner and can be used to control load in
the same way as cutting a burner in and out.^2 When overbed
feeding is used, the fi ne material in the fuel has a tendencyto elutriate too swiftly. If the fuel is fed underbed, the fi nes
will have a longer residence time. Excess CO generation can
result with the excessive burning of fi nes. This in turn can
lead to overheating which could cause superheater controls
to trip-off. Ash-slagging is another potential problem asso-
ciated with overheating. Sometimes it may be necessary to
recycle the fl y-ash in order that carbon is more thoroughly
burned and sorbent more completely utilized.
In-bed combustor tubes are generally used to extract heat
(create steam). The heat transfer coeffi cient range is higher
than that of CFB’s, i.e., 40–70 Btu/hr.ft^2 F. Erosion of the
tubes is a problem which is ever present in the bubbling bed
combustor. The problem worsens as bed particle velocity
increases. Horizontally arranged tubes are more susceptible
to erosion than are vertical tubes. Various methods of erosion
protection include metal spray coatings, studding of the tube
surfaces with small metal balls, and wear fi ns. Occasionally
recycled cold fl ue gas is used in lieu of tubes.
Waterwalls located in the upper portion of the combustor
are also used (as with CFB’s) to extract heat. The lower portion
is refractory lined. Combustor free-board is usually between
15 and 30 ft. The typical convective pass, cyclone, air heater,
particle separator scenario closely resembles that of the CFB.C. Pressurized Fluidized Bed
Combustors (PFBCs)The pressurized fl uidized bed combustor is essentially analo-
gous to the bubbling bed combustor with one exception—the
process is pressurized (10 to 16 atmospheres) thereby allow-
ing the fl ue gas to drive a gas turbine/electric generator. This
gas turbine along with a stream-driven turbine creates a very
effi cient “combined cycle” arrangement.
PFBCs may also be “turbocharged,” i.e., before the
fl ue gas enters the gas turbine, heat is extracted via a heat
exchanger. Steam created by the energy transfer is used to
drive the compressor which pressurizes the system. There is
no energy excess to drive an electric generator in this case.
Deeper beds (typically 4 m.) may be used in PFBCs
because they are pressurized. The residence time of a parti-
cle in the bed is longer than that of a particle in the shallower
bed of a bubbling bed combustor. The fl uidizing velocity
(typically 1 m/s) is also lowered because of pressurization.
As mentioned before, lower velocities minimize the amount
of in-bed tube erosion.
Two other benefi ts of pressurization are a reduced bed
cross-sectional area and reduced boiler height.
Since combustor effi ciency and sorbent utilization are
excellent, recycle is rarely needed. However, when very unre-
active fuels are burned, recycling of fi nes may be necessary.
Since PFBCs are pressurized, certain design character-
istics must be taken into consideration, especially in regard
to the gas turbine. This turbine supplies the combustion and
fl uidizing air for the bed. Unlike conventional AFBC’s the
turbine inlet air is dependent upon certain temperature and
pressure conditions since this inlet air is actually the exhaust
gas from the combustor. To compensate for variations in load
and subsequent changes in the exhaust gas conditions the gasC006_001_r03.indd 403C006_001_r03.indd 403 11/18/2005 2:28:15 PM11/18/2005 2:28:15 PM